This invention relates to a dual-phase stainless steel exhibiting improved resistance to corrosion caused by nitric acid, and particularly to such a dual-phase stainless steel as that used for structural members in the construction of an apparatus for chemically reprocessing spent nuclear fuels.
Chemical treatment of the spent nuclear fuel of light-water reactors is carried out under high temperature, nitric acid-containing environments, and such 25% Cr-20% Ni base alloys as URANUS 65 (tradename) have been used as a structual material therefor. However, the degree of corrosion resistance which 25% Cr-20% Ni base alloys exhibit is not satisfactory under medium or high concentrations of nitric acid or when the corrosive environment further contains Cr.sup.6+ ions. It has also been proposed to use 17% Cr-14% Ni-4% Si base steels and 8% Cr-20% Ni-6% Si base steels under such highly corrosive environments, although these materials do not exhibit satisfactory resistance to corrosion even under conditions containing high or medium concentrations of nitric acid, either. Even more they do not exhibit corrosion resistance under environments where Cr.sup.6+ ions are also contained, since the Cr.sup.6+ ions act as an oxidizing agent to markedly accelerate the intergranular corrosion.
Dual-phase stainless steels such as 27% Cr-8% Ni-0.1% N base alloys have been proposed as steels highly resistant against nitric acid (See Japan Laid-Open Patent Specification No. 31068/1983). However, silicon is added in an amount of up to 2% merely as a deoxidizing agent and they do not exhibit satisfactory resistance under corrosive conditions containing an oxidizing agent such as Cr.sup.6+ ions.
Thus, a metallic material which exhibits satisfactory levels of corrosion resistance in the presence of Cr.sup.6+ ions in nitric acid solutions has not yet been developed.
Now many nuclear power plants are in operation, and a relatively large amount of the total power supply has come from light-water nuclear reactors. It has also been necessary to reprocess a large amount of the spent nuclear fuels from these reactors with nitric acid solutions. What this means is that there is a need in the art for a material which can exhibit improved resistance to corrosion under nitric acid-containing environments. It is also required that structural members for an apparatus used in reprocessing spent nuclear fuels, having a long, continuous service life be provided.
Materials and articles made thereof which meet the above needs should satisfy the following requirements:
(1) First, they must exhibit improved resistance to corrosion, particularly to corrosion by nitric acid;
(2) Second, they must also exhibit satisfactory resistance against any increase in corrosion rates or acceleration of intergranular corrosion, which are caused by increases in corrosion potential due to contamination from Cr.sup.6+ ions or from an oxidizing agent from nuclear fuels such as Ru; and
(3) Third, they must suppress any degradation in the corrosion resistance of welds by avoiding becoming sensitized during welding. This is because welding is widely used in the construction of these apparatuses.